In recent years, progress is being made in research and development of diverse functional elements which involve the use of an organic semiconductor.
Organic EL elements are known as being one of the most common among such functional elements. An organic EL element is a current-driven light-emitting element, and commonly has a pair of electrodes, namely a pixel electrode and a common electrode, and for example a hole injection transport layer, a light-emitting layer and an electron injection transport layer, layered between the pair of electrodes.
When driving the organic EL element, voltage is applied between a pair of electrodes. The emission of light from the organic EL element is caused by an electric-field light-emitting phenomenon taking place as a result of the transition to the ground state of excitons generated by the recombination of holes, which are injected from the anode to the hole injection transport layer, and electrons, which are injected from the cathode to the electron injection transport layer. Now more attention is being given to the application of organic EL elements as a light emitter or a light source for various display devices. This owes to the advantageous characteristics of the organic EL element, which include: high visibility resulting from self-luminescence; and excellent shock resistance resulting from the fully solid-state structure thereof.
Organic EL elements can be largely divided into two types, according to the material used for forming the light-emitting layer therein. The first type of organic EL elements is a vapor deposition type. In a vapor deposition-type organic EL element, mainly organic low molecular material is formed as a film by applying a vacuum process such as a vapor deposition method. The second type of organic EL elements is an application type. In an application-type organic EL element, mainly either organic high molecular material, or organic low molecular material having excellent thin film forming properties is formed as a film by applying a wet process such as an inkjet method and a gravure printing method.
When comparing the two types of organic EL elements, vapor deposition-type organic EL elements have gained more popularity up to this point, for reasons such as higher light-emitting efficiency of the light-emitting material and longer operating lifetime in comparison with application-type organic EL elements (for instance, refer to Patent Literatures 1 and 2). As a result, vapor deposition-type organic EL elements have already been put into practical use in mobile phone displays, small-sized TVs and the like.
At the same time, much effort is also being made in research and development of technology for enhancing the light-emitting efficiency of organic EL elements. Efficient injection of carriers (holes and electrons) from the electrodes to the functional layer is essential in causing an organic EL element to emit light with high efficiency, high luminous intensity, and low electrical consumption. For the efficient injection of carriers, it is effective to provide the hole injection transport layer and the electron injection transport layer, and reduce the energy barrier (injection barrier) between each electrode and the light-emitting layer.
Considering the above, the electron injection transport layer is commonly made of an alkaline-earth metal such as Ba.